双层刚柔组合搅拌桨调控流体宏观不稳定性行为

流体宏观不稳定性是搅拌槽内流体流动存在大尺度低频非稳态准周期现象,可以影响流体的能量﹑质量的传递行为。为揭示在双层组合桨作用下搅拌槽内流体的非稳态流动规律,实验采用频谱分析和流场可视化技术研究双层组合桨搅拌槽内自来水体系的宏观不稳定性,对比分析了双层刚性桨和双层组合桨对流体混合的影响。结果表明:直径为T的搅拌槽内流体宏观不稳定频率与转速呈线性增大趋势,在转速为180 r·min^-1时离底距离 0.25T刚柔组合桨体系的宏观不稳定性频率消失,出现谱带现象,流场呈现多尺度结构特征,而离底距离为0.33T和0.5T的刚柔组合桨体系的宏观不稳定性频率分别为0.5096 Hz和0.3459 Hz。双层组合桨体系分别使流体的混合时间缩短了22.5%和35%左右,减小离底距离,可使流场的规则区减小。双层刚柔组合桨调控流体宏观不稳定性,强化流体的能量传递行为,从而缩短混合时间,提高了流体的混合效率。

Macro-instability regulated by double rigid-flexible combination impeller

Fluid macro-instability is a large scale low frequency transient quasi-periodic phenomenon existing in stirred vessel, and affects fluid energy and mass transfer behavior. To reveal the nature of unsteady flow in a stirred vessel with double rigid-flexible impeller, tap water was used as working fluid, and wavelet analysis and flow field visualization were adopted to analyze the influence of double rigid-flexible impeller on the frequency of macro-instability. The difference between double rigid and double rigid-flexible impellers was analyzed in terms of mixing performance. The frequency of macro-instability increased linearly with agitation speed. At agitation speed 180 r·min^-1, macro-instability frequency values of rigid-flexible impeller were 0.5096 Hz and 0.3459 Hz with off-bottom clearance 0.33T and 0.5T, respectively. Compared with double rigid impeller system, mixing time was shortened by about 22.5% and 35%, respectively. But macro-instability with off-bottom clearance 0.25T disappeared and bands phenomenon appeared. Shortening the off-bottom clearance could reduce the regular area. Double rigid-flexible combination impeller regulated fluid macro-instability and strengthened energy transfer behavior, so as to shorten mixing time and improve mixing efficiency of the fluid.